Rendering one pass only to avoid unnecessary computations in Cycles
Cycles works by tracing many light paths through your scene and gathering information (depth, normals, color, etc) from the interactions of those paths with stuff in the scene. Simply ignoring the color information won't give you any significant performance gain.

All you can do1 is limit the number of paths you have to trace as much as possible; set the bounces to 1 and the samples as low as you care to go. You probably still want a few samples in order to retain a bit of anti-aliasing (16 should be plenty). If you have lots of motion blur or DoF (or any other situation where a path straight out of the camera will not always give the same value), then you may need more.

1: Without switching to a different engine which might better suit your needs, that is.